The human lysosomal storage disease galactosialidosis has been characterized as a combined deficiency of b-galactosidase and neuraminidase enzymatic activities. Clinical symptoms include neurological-, skeletal- and eye abnormalities, in the more serious cases mental retardation and early death. It has been established that the primary defect in this disease is in fact the deficiency of a third lysosomal protein, the "Human Protective Protein" - HPP. The protective protein is thought to be involved in a multi-enzyme complex with b-galactosidase and neuraminidase, preventing the degradation of the former and providing stabilization/activation of the latter, in the harsh acidic environment of the lysosome. Our goal is to determine the 3-dimensional structures of the precursor and mature forms of the protective protein, by X-ray diffraction analysis of protein crystals.